The long-term objective of the proposed study is to define the participation of lymphocytes in leukemogenesis and to establish whether or not alterations of the immune system may occur before or during the development of acute lymphocytic leukemia (ALL). The specific aims are: (1) To determine the role of human peripheral blood leukocytes and bone marrow cells in the proliferation and differentiation of immunocompetent cells in "in vivo" and "in vitro" cultures. (2) To compare the response to antigenic stimulation of peripheral blood lymphocytes and bone marrow-derived cells from patients with ALL in relapse with the response of cells from patients without ALL and with ALL in remission. (3) To establish whether alterations of the immune mechanisms may be the first indication of relapse in children with ALL in remission that are not receiving immunosuppressive therapy.